Sonar transducers are employed to provide and detect acoustic signals, usually in a fluid such as water. While a variety of configurations of sonar transducers have been used, one existing design is referred to as the tonpilz configuration. In this configuration, the transducer has a tail mass at a proximal end having connections for signals and a source of electrical power, a head mass at a distal end and a stack of ring shaped drivers, such as piezoelectric ceramic elements, electrically connected in parallel, extending longitudinally between and in physical contact with the head mass and the tail mass. A tie rod maintains the stack of drivers under compressive stress. Excitation of the drivers at a frequency of resonance causes the head and tail masses to oscillate longitudinally at a longitudinal frequency of resonance to provide a sonar signal.
Transducers in the longitudinal or tonpilz configuration described above typically have a resonant frequency above about two kilohertz. In these frequency ranges, signals become severely attenuated at long ranges. As attenuation is approximately proportional to the square of the frequency, for a given power level, longer-range communications can be obtained only by providing lower-frequency signals. Lower-resonance frequencies can be obtained by increasing the mass of the head mass and tail mass. However, the useful bandwidth of the signal is roughly inversely dependent on the equivalent mass of the combined head and tail mass. Thus, obtaining a lower frequency by increasing the mass of the head mass and tail mass results in a decreased bandwidth. The bandwidth at low frequencies is roughly proportional to the amount of radiation resistance, which in turn is proportional to the square of the radiating area for low frequency transducers. Thus, at low frequencies, transducers having an increased area at the face of the head mass provide a greater useful bandwidth. However, increasing the area at the face sufficiently to compensate for decreased bandwidth at low frequencies may result in undesirably large transducers with poor beam characteristics, such as excessively narrow beamwidth and deleterious grating lobes.